Caving consideration to the current trend for increasing development of portable wireless electronic products, there is a strong need for development of a secondary battery having a high energy density in order to achieve miniaturization and weight reduction of these electronic devices and instruments.
As the secondary battery, there have been used lead acid batteries, nickel cadmium (Ni—Cd) batteries, nickel metal hydride batteries, and the like. In recent years, lithium ion batteries are widely used due to the light weight and high energy density, since it was first commercialized in 1991.
As a cathode material for the lithium ion battery, lithium cobalt oxide (LiCoO2) basically having a layered rock-salt structure is currently the most widely used material, and has been recognized as the most important ingredient constituting the lithium ion battery (G.-A. Nazri and G. Pistoia, “Lithium Batteries”, Kluwer Academic Publishers, 2004).
However, the lithium cobalt oxide (LiCoO2) suffers from disadvantages such as relative expensiveness of a cobalt (Co) material per se, as compared to other transition metals such as iron (Fe), manganese (Mn), nickel (Ni), and so on, and environmental harmfulness. For these reasons, there is a continuing attempt to develop a solid solution oxide which is synthesized with replacement of Co with Mn or Ni.
Further, a layered oxide material such as the aforementioned LiCoO2 has a shortcoming associated with deterioration of the structural stability, due to the release of oxygen which occurs upon intercalation and deintercalation of lithium ions.
Further, the LiCoO2 suffers from a very high risk of explosion, when a particle size of the lithium cobalt oxide decreases to a nanoscale level, so there has been continuously raised fundamental problems associated with safety concerns. Therefore, there is a limit in production and utilization of nanoscale particles having a large specific surface area in order to obtain excellent output characteristics.
As discussed above, with various problems associated with the use of the LiCoO2 as a cathode material, and in order to secure the battery safety via prevention of probable explosion due to overcharge or overheating while achieving a higher energy density, a great deal of research and study has been focused on development of a novel synthesis method or a novel cathode material which is totally different from a conventional oxide material. Among other things, lithium transition metal phosphates with an olivine structure (LiMPO4) (M=Fe, Mn, Co, Ni, Ti, Cu or any combination thereof) are attracting a great deal of interest as a next-generation cathode material in the future (J.-M. Tarascon and M. Armand, Nature, Vol. 414, p. 359 (2001)).
Since the first experimental demonstration, made by Goodenough et al at The University of Texas (A. K. Padhi, K. S, Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc., Vol. 144, p. 1188 (199′7)), reporting that the electrochemical intercalation and deintercalation of lithium ions can take place in LiFePO4 which is a kind of the aforesaid LiMPO4, numerous research groups and institutions have proposed various synthesis methods of LiMPO4 in conjunction with intensive research for practical application of LiMPO4 as a cathode material.
Generally, the LiMPO4 can be represented by a general formula of M′M″(XO4) (M′ and M″=a metal cation including a transition metal, and X=P, S, As, Mo, Si or B), and the crystal structure thereof is identical with that of the mineral olivine [(Mg,Fe)2 (SiO4)]. In the LiMPO4, lithium (Li) is positioned in an oxygen octahedral interstitial of the M′ site, and the transition metal (M) is positioned in an oxygen octahedral interstitial of the M′ site, thereby forming an ordered olivine structure. Among the LiMPO4, LiFePO4 and Li(Fe,Mn)PO4 compounds are natural minerals already well known as Triphylite.
A cathode material for a lithium ion battery should be thermally and chemically stable. Referring to the crystal structure of the LiMPO4, phosphorus (P) and oxygen (O) participate in the formation of a strong covalent bond to thereby result in a tetrahedral structure, so the LiMPO4 advantageously exhibit excellent thermal and chemical stability, unlike conventional layered oxide materials suffering from a poor structural stability due to release of oxygen which occurs upon intercalation/deintercalation of lithium ions. The lithium transition metal phosphate (LiMPO4) has a structure in which octahedra of the transition metal (M) and oxygen (O) are connected in a one-dimensional chain structure. Further, due to the ordered one-dimensional arrangement of lithium ions within another oxygen octahedron in the Y-axis direction, upon viewing the structure of LiMPO4 on the rectangular coordinate system, it is predicted that the mobility of lithium ions in the Y-axis direction will be very high (D. Morgan, A. Van der Ven, and G. Ceder, Electrochem., Solid-State Lett., Vol. 7, p.A30 (2004)).
Further, as another important technical factor required for utilization of the LiMPO4 as the cathode material for the lithium ion battery, the electrical conductivity should be taken into account. Generally, an electrode material employed in the lithium ion battery is essentially required to have an excellent electrical conductivity in order to achieve minimization of polarization effects arising from the redox reaction and an effective supply of electrons to the connected external resistance.
The LiFePO4 has an electrical conductivity of less than 10−9 S/cm and is therefore substantially an insulating material at room temperature. Upon considering the fact that an electrode material employed in the lithium ion battery is required to have an excellent electrical conductivity in order to achieve minimization of polarization effects arising from the redox reaction and an effective supply of electrons to the connected external resistance, it was revealed that the LiFePO4 suffers from a fatal disadvantage in that it is not suitable for practical application as the cathode material in the lithium ion battery, despite excellent crystallographic, thermal and chemical properties.
A variety of experimental approaches have been attempted to overcome the above-mentioned disadvantages and problems. For example, mention may be made of a method of coating the surface of the already-synthesized LiFePO4 particles with a variety of carbon materials including graphite having an excellent electrical conductivity to thereby decrease the resistance between particles, thus providing a path through which electrons can be conducted sufficiently (H. Huang, S. C. Yin, and L. F. Nazar, Electrochem. Solid-State Lett., Vol. 4, p.A170 (2001); and Y.-H. Huang, K.-S. Park, and J. B. Goodenough, J. Electrochem. Soc., Vol. 153, p.A2282 (2006)), or a method of improving an electrical conductivity of an electrode material by combined addition of small silver or copper metal particles upon synthesis of LiFePO4 particles (F. Croce et al., Electrochem. Solid-State Lett., Vol. 5, p. A47 (2002)).
However, most of the aforementioned conventional approaches and attempts suffer from disadvantages such as additional coating processes, and addition of non-active materials which have no relation to the electrode material during a production process. Accordingly, there is a need to find alternative methods of research on LiMPO4.